A large user story that awaits decomposition into smaller stories prior to implementation. When an epic story works its way up the backlog, it is usually so decomposed. Epics are sometimes far off on the development horizon and have lower priority.
In the scrum framework stories that are sprint ready must be small enough that they can be confidently implemented within the timebox of a single sprint.
NB: One can easily be tempted to associate the term ‘epic’ with importance; in Agile, epic relates only to size.
Background of the Term
As it relates to agile methodologies, Mike Cohn coined the term epic in his book User Stories Applied: For Agile Software Development. Chapter 2 of that book is available here as a pdf, in which there is discussion of epics.
Agile Alliance | Glossary | Epic
Don’t Finish Your Epics! Deliver More Value Instead.
User Stories Applied: For Agile Software Development